Methamphetamine is an addictive and powerful stimulant of the central nervous system as well as the peripheral nervous system. It is similar in action to amphetamine, but is more potent. Both drugs are derivatives of phenylethylamine (see FIG. 1), as are the designer methamphetamine class of drugs including ecstasy (3,4-methylenedioxymethamphetamine, MDMA), (+/−)-N-methyl-1-(3,4-methylenedioxyphenyl)-2-butanamine (MBDB) and (+/−)-3,4-methylenedioxyethylamphetamine (MDEA) and others. Due to their powerful stimulant effect, these drugs are heavily abused.
Such drugs are normally detected in urine, blood, sweat, oral fluid and hair. Different types of immunoassays are available for this purpose. The assays generally utilise antibodies (polyclonal and monoclonal) raised to amphetamine and methamphetamine by immunising animals with drug derivatives attached to a carrier protein (e.g. bovine serum albumin) via a functional group attached to the phenyl group of the drug, in particular the para-position. The immunoassay will then utilise these antibodies and the same drug derivatives linked to a label, which may be, for example, a protein, an enzyme, a radioactive label or a fluorescein label. Prior art publications in this area covering both the production of antibodies and labelled drug derivatives are numerous and related commercial immunoassays are available like the Abbott and Roche systems (EP 0371253, EP 0279213, EP 1167976, U.S. Pat. No. 4,329,281, U.S. Pat. No. 4,041,076, U.S. Pat. No. 4,067,774, U.S. Pat. No. 3,996,344, U.S. Pat. No. 4,016,146 and WO 02057739). Other ways of making specific antibodies utilise drug derivatives attached to a carrier protein via the aliphatic alkyl-end (utilising 4-aminobutyl amphetamine and 4-aminobutyl methamphetamine) (EP 0359063 and Analytical Biochemistry, 1999, volume 274, pages 118-124). The immunoassay is set up using this antibody and the same derivative attached to a label. Assays specific for the ecstasy-class of drugs have also been reported. US Patent 2003207469 assigned to Microgenics Corporation describes the use of an ecstasy drug analogue, a derivative of 2-amino-methylenedioxyphenyl, for attachment to a carrier protein for immunisation and a for production of labelled analogue.
It is desirable that immunoassays for methamphetamine and the ecstasy class of drugs show specificity for the illicit drugs. This is important as many phenylethylamine analogues are used in over-the-counter medicines like decongestants (cough mix). The list is large and includes: ephedrines, pseudoephedrines, phenylpropanolamine and phenylepherine. Some of the popular medicines are Sudafed®, Contact®, Vicks® inhaler and Primatine® tablets. In addition, compounds like tyramine (4-hydroxy-phenethylamine) (see FIG. 1) can be present naturally in biological samples being tested for amphetamines/methamphetamines. Immunoassays should desirably have less than 0.4% cross-reactivity with tyramine to be useful (EP 0371253). Presently known immunoassays utilising derivatives of ecstasy have the limitation of not being able to detect methamphetamine, especially at the low levels that may exist in biological fluids other than urine (e.g. oral fluid). Immunoassays based on using antibodies to the drug derivatives attached to a carrier protein via the phenyl ring suffer from lack of specificity for methamphetamine and are subject to interference by many of the licit drugs mentioned above. The Abbott assay requires prior sample treatment to eliminate or reduce undesirable cross-reactivity. Samples are treated with sodium periodate to eliminate cross-reactivity with drugs like ephedrines, pseudoephedrine and phenylpropanolamine as well as tyramine (EP 0371253). Over 40 medicines are listed as causing false positive results in methamphetamine urine tests (www.erowid.org/).
The lack of specificity of amphetamine/methamphetamine immunoassays has become a matter of concern (Journal of Analytical Toxicology, 2003, volume 27, pages 265-269, and references within). A recent study (Journal of Forensic Science, 2004, volume 49, pages 160-164) evaluating commercially available test kits for interference by ephedrine, pseudoephedrine and phenylpropanolamine showed an un-acceptable level of interference and concluded that manufacturers underestimate the level of undesirable cross-reactivity shown by their kits.